1. Field of the Invention
The present invention relates to a multi-beam optical scanning method and system, and more particularly to a multi-beam optical scanning method and system capable of performing a high speed scanning and a high image resolution. Also, the present invention relates to an image forming apparatus having the above-mentioned multi-beam optical scanning system.
2. Discussion of the Background
Optical scanning apparatuses are widely used as image recording devices in image forming apparatuses including laser printers, digital copying machines, facsimile machines, and so forth. In such an optical scanning apparatus, many attempts to increase the scanning speed and image resolution have been conducted. For example, Japanese Laid-Open Patent Application Publication, No. 2001-228422, describes a multi-beam optical scanning apparatus in which multiple light beams and an aperture element are arranged between a collimate lens and an optical deflection mechanism relative to both light bundles in converging and parallel forms. In this publication, a plurality of diverging light beams emitted from a multi-beam semiconductor laser array diode used as plural light sources are converted by the collimate lens into approximately a parallel laser light beam bundle or a converging laser light beam bundle and are incident to a cylindrical lens. When the laser light beam bundle is output from the cylindrical lens, the laser light beam bundle has a form approximately as it is in a main scanning plane but has a converging form in a sub-scanning plane. The laser light beam bundle is then limited both in the main scanning and sub-scanning directions by the aperture element.
The laser light beam bundle limited by the aperture element forms in focus a linear image extending in the main scanning direction on a deflection surface of a light deflecting mechanism (e.g., a polygon mirror). The laser light beam bundle including the plural laser light beams are deflected by the deflection surface of the light deflecting mechanism and pass through an fθ lens system so that the plural laser light beams form in focus images of spots on a surface of a photosensitive drum. By rotating the light deflecting mechanism in a predetermined direction, the images of spots start at a same time to optically-scan the surface of the photosensitive drum at a constant speed in the main scanning direction. Thus, an image recording is performed and an image is recorded on the surface of the photosensitive drum.
For another example, Japanese Laid-Open Patent Application Publication, No. 10-246860, describes another multi-beam optical scanning apparatus in which an aperture element is arranged at a position close to a collimate lens in a case of a converging light beam bundle. In this case, the light beam bundle having the plural light beams emitted from a multi-beam semiconductor laser array diode is converted approximately into a parallel light beam bundle by the collimate lens and partially enters into the aperture element positioned close to the collimate lens. Then, the thinned parallel light beam bundle passes through a cylindrical lens. Being output from the cylindrical lens, the laser light beam bundle has the parallel form approximately as it is in a main scanning plane but has a converging form in a sub-scanning plane.
The laser light beam bundle then forms in focus a linear image extending in the main scanning direction on a deflection surface of a light deflecting mechanism (e.g., a polygon mirror). The laser light beam bundle including the plural laser light beams are deflected by the deflection surface of the light deflecting mechanism and pass through an fθ lens system having refractive powers opposite to each other so that the plural laser light beams are led to positions different from each other to form in focus images of spots on a surface of a photosensitive drum. By rotating the light deflecting mechanism in a predetermined direction, the images of spots are started at a same time to optical-scan the surface of the photosensitive drum at a constant speed in the main scanning direction. Thus, is performed and an image scanned onto the surface of the photosensitive drum.
However, the following requirements need to be solved to achieve a high speed scanning and a high image resolution at a relatively high level. That is, a first requirement is to reduce a variation in a beam spot diameter against a temperature change. A second requirement is to capture a sufficient amount of light for high speed scanning. A third requirement is to control the beams spot diameter. A fourth requirement is to reduce the deviation in relative dot positions for multiple light beams on a scanned surface in the main scanning direction, and reduce the deviation in the deviations in dot positions in the main scanning direction due to an unevenness of reflection points in a surface of the polygon mirror. A fifth requirement is to reduce the adverse affect of ghost light generated by a reflection of light beams with the aperture element and the cylindrical lens.